The Wideband Code Division Multiple Access (WCDMA) cellular system specified by the 3rd Generation Partnership Project (3GPP) provides a variety of location based services. These services all utilize the location of a user equipment (UE) for some purpose. Currently the most important service is the E-911 emergency positioning functionality that is regulated for the US market. E-911 phase 2 positioning requirements state that all cellular networks have to be able to position users within 30s, with accuracies better than 150 m (95%) and 50 m (67%), as counted for each county and each emergency center (PSAP). Considering the fact that the Global Positioning System (GPS) has very limited coverage indoors and the fact that most cell phone calls are today placed indoors, these are very difficult requirement. The consequence is that the WCDMA cellular system standardizes not only GPS, but actually assisted GPS (A-GPS) which is an enhancement of the GPS system. On top of that a number of alternative positioning methods that rely on cellular network measurements are standardized in WCDMA.
It can be noted that emergency positioning requirements are also under way in other regions, like e.g. India, However, in markets outside the US, cell phone positioning is primarily used for commercial location based services such as personal and vehicular navigation, friend finding and geographical search services. Lawful surveillance and intercept are other situations where location technology is particularly useful.
To support the positioning methods the whole cellular infrastructure is prepared for processing and signalling of geographical position information. In WCDMA most of the positioning related functionality resides in the radio access network (RAN) or closely associated with it, i.e. in the radio network controller (RNC) or the Stand-Alone Serving Mobile Location Centre (SAS) node, in the node B (the base station), and in the UE. The core network (CN) is mostly involved with the signalling of geographical information.
The cell ID positioning method that determines the terminal position with cell granularity represents the back bone low end approach in the majority of the cellular systems, including WCDMA. The method has the advantage of an instantaneous response and availability wherever there is cellular coverage.
Due to the advantages of the cell ID method, attempts have been made to enhance the accuracy, while maintaining the benefits. The most common way is to augment the cell ID method with an assessment of Round Trip Time (RTT), i.e. the travel time back and forth between the terminal and the base station. Together this defines a truncated circular strip around the base station.
In areas with relatively low expected density of WCDMA user equipments, the DL coverage, rather than the DL capacity, is typically the limiting factor. The base station and in particular the high power parts thereof are expensive parts of the communication radio access network and in order to save costs, a so-called psi-coverage configuration can be used. In such a configuration, a single node B is connected to three standard cross-polarized sector antennas. The DL signals are split into the three antennas. The connection is made via two specially designed 3-way splitter-combiners. In this way, a maintained DL coverage can be obtained, but with one node B instead of three.
However, to maintain the performance, the UL signals are received from multiple antenna branches and combined into one radio, using different frequencies for different sectors, thereby reducing interference and noise. The interference between the combined signals originating from different antennas can therefore be reduced. This results in an UL sensitivity comparable to an ordinary 3-sector configuration. Compared to an omni configuration the UL capacity is substantially improved.
Psi-coverage thus maps the DL and UL into one omni sector-cell, so the RNC views the configuration as a high capacity and high coverage omni sector-cell. In other words, the psi-coverage approach involves an omni base station with three antennas, stripped of some but not all three sector functionality. It is intended to have a very low cost, and good coverage. This allows the normal functioning of most RAN features. However, the positioning capabilities are limited, as only one cell area is defined in the RNC per omni sector-cell. In other words, the sector information is lost for positioning. The effect on RTT positioning is that the angular extension of the ellipsoid arc will be 360 degrees rather than about 120 degrees and the regions where the UE may be located becomes 3 times as large as if the sector of the UE would be known to the position calculation function of the RNC or SAS node. This represents a huge increase of the inaccuracy of the RTT positioning method, with a factor of 3. For such systems, the E-911 phase 2 positioning requirements become even more difficult to fulfil. Even though the node B has available auxiliary positioning information, a considerable rearranging of the RNC is today necessary for allowing such information to be utilised by positioning nodes in or connected to the RNC. Such rearranging is expensive and complex.